1. Field of the Invention
The invention relates to a pneumatically actuatable linear drive for automatic spot welding machines or robots having a plurality of pistons guided in a tandem configuration on a piston rod and disposed in a cylinder which is closed at one end by a cylinder bottom and has a guide ring penetrated by the piston rod at its other end region; between each two adjacent pistons is an intermediate bottom attached to the cylinder and penetrated by the piston rod, so that the cylinder is divided into at least three pressure chambers, namely a first pressure chamber located between the cylinder bottom and the first piston adjacent to it, a second pressure chamber located between the first piston and the intermediate bottom adjacent to it, and a third pressure chamber located between the second piston and the intermediate bottom; the piston rod includes a segment extending between the pistons having an axial conduit with an end oriented toward the cylinder bottom having a check valve and discharging through the check valve into the first pressure chamber, the axial conduit can be connected through radial conduits to the first and the third pressure chambers, the cylinder has two air line connections, the first of which leads into the first pressure chamber and can communicate with the third pressure chamber, while the second of which leads into the second pressure chamber through a throttle valve; the first piston is axially movably disposed on an end segment of the piston rod having a lesser diameter, and in its end position toward the cylinder bottom it closes the radial conduits leading out of the axial conduit of the piston rod into the first pressure chamber, while in its terminal position remote from the cylinder bottom it opens these conduits.
2. Description of the Related Art
A linear drive having such characteristics is known from German Published, Non-Prosecuted Application DE 35 42 069 Al. The known apparatus has the object of braking an electrode motion in a direction toward a workpiece in order to reduce production of noise upon application of the electrodes. The time loss brought about by the delaying procedures is negligibly low.
In the known apparatus, this object is intended to be attained by a piston 7 adjacent to a second pressure chamber 20 being axially displaceable over a reduced diameter region 9' of a shaft 9 as a function of a pressure difference between a first and the second pressure chambers 18, 20. The displacement is between a first position, in which it covers a connecting line 17 surrounding a valve 15, between a conduit 16 and the second pressure chamber 20, and a second position, in which it uncovers the connecting line 17. In its second position, it rests on a shoulder 10 of the shaft 9.
Although the stated object can be attained by such a piston-cylinder configuration, nevertheless, particularly in the embodiment shown and described in the specification as an exemplary embodiment, it has a disadvantage which is that it is structurally very complicated and therefore very expensive to manufacture. Another disadvantage is that the throttle configuration located laterally of the cylinder considerably increases the total amount of space required for the linear drive of the welding electrode. That has a disadvantageous effect in terms of the possible uses of the welding robot, because welding tongs, with their drive mechanism, can no longer be guided through very tight spaces.
A functional disadvantage is that in order to reach its initial position in which it closes off the radial connecting conduits from the first pressure chamber to the axial conduit of the piston rod, the piston, which is axially movable over an inner end segment of the piston rod, is completely dependent on the pressure difference between the first and second pressure chambers. If this pressure difference should not suffice to displace the piston into its end position toward the cylinder bottom, then when the next working stroke begins both pistons may be acted upon simultaneously, so that the damping effect that is sought is prevented.
It is accordingly an object of the invention to provide a pneumatically actuatable linear drive for automatic spot welding machines, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which does so in such a way that the structure is simpler, while cast parts are avoided and reliable functioning is assured.